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Growth and nitrogen fixation were followed during the life cycle of Setaria italica (foxtail millet) inoculated with Azospirillum brasilense in controlledenvironment growth chambers. The plants were fertilized at seeding with a limiting amount of combined nitrogen and maintained with an N-free mineral solution. During maturation of the plants, substantial nitrogenase activity, measured by acetylene reduction, developed in the rhizosphere, with total fixation estimated to be equivalent to 20% of the N in the inoculated plants. The peak of this activity coincided with depletion of soluble nitrogen from the system, which in turn was reflected by a sharp decrease in the nitrate reductase activity of the leaves. A. brasilense was found in association with the root populations at 8 x 107